Active pixel sensor (APS) devices are described in U.S. Pat. No. 5,471,515. These devices include an array of pixel units that convert light energy into electrical signals. Each pixel includes a photodetector and one or more active transistors.
Unlike charge coupled devices (CCD), APS imaging devices are compatible with complementary metal oxide semiconductor (CMOS) technology. This feature enables integration of imaging electronics and digital image signal processing electronics on the same chip in APS devices. Such integration can provide advantages in design and performance.
Conventional CMOS imaging devices can be prone to producing artifacts in the resulting image. Artifacts are objects in the image that appear black when in fact they should be the brightest regions of the image.
Artifacts can occur in oversaturated pixels in the APS. Oversaturated pixels are pixels that are exposed to more light energy than the pixel""s photodetector can absorb during exposure time.
This is mainly a problem in very high contrast images. The extreme brightness of the sun will produce artifacts in virtually all CMOS imagers, even at very short integration times. This is a disconcerting effect in outdoor imaging situations, in which the sun is often accidentally included in the field of view.
FIG. 1 illustrates such a situation in a digital image 10 produced using a conventional APS CMOS imager. The presence of the sun 12 in the image produces an artifact 14 in which the center of the sun appears black.
An active pixel sensor according to an embodiment includes at least one pixel having a photodetector that outputs an output level indicative of incoming light. A storage element stores said output level during a photodetector readout operation. A comparison element compares the output level to an adjusted saturation voltage. An output selector electrically connected to the comparison element selectively outputs an adjusted maximum voltage during a pixel readout operation in response to the comparison element determining that the adjusted saturation voltage exceeds the output level.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.